Wild female blackflies were collected at five cattle sheds in Oita, southern Japan, where a human zoonotic onchocerciasis had occurred. Among the eight Simulium species captured, S. bidentatum was predominant, followed by S. arakawae. Natural infections with filarial larvae were found in 10.5-20% of the parous S. bidentatum collected at three cattle sheds, and also in 6% of the parous S. arakawae captured at one cattle shed. Only one infected S. arakawae harbored two third-stage larvae in the thorax while the others had 1-4 either first-or second-stage larvae in the thorax. Third-stage larvae obtained from wild flies maintained alive for 6-9 days after being collected are distinguished into three types of Onchocerca : type I recovered from S. bidentatum, S. arakawae and S. aokii is probably new, having an elongated body length (1, 075-1, 380 pm), and types II and III, recovered from S. bidentatum and S. arakawae respectively, are each indistinguishable from O. gutturosa and O. lienalis. It is suggested that due to its anthropophily as well as zoophily S. bidentatum is the most probable vector of zoonotic Onchocerca (either type I or II) which may be acquired by humans in the western suburbs of Oita.